Recently, various standards have been developed for data communication over broadband wireless links. One such standard is set out in the IEEE 802.16 specifications and is commonly known as WiMAX. The specifications include IEEE 802.16-2004, primarily intended for systems having fixed subscriber stations, and IEEE 802.16e-2005 which among other things provides for mobile subscriber stations. In the following description, the abbreviation MS is used as shorthand for both mobile and fixed subscriber stations. The term “user” is also used equivalently to mobile or fixed subscriber station.
The entire contents of IEEE Std 802.16-2004 “Air Interface for Fixed Broadband Wireless Access Systems” and IEEE Std 802.16e-2005 “Amendment 2 and Corrigendum 1 to IEEE Std 802.16-2004” are hereby incorporated by reference. IEEE 802.16 defines wireless communication systems in which the mobile stations communicate with a base station within range, the range of a base station defining at least one “cell”. By deploying base stations at suitable positions within a given geographical area, and/or by providing multiple antennas in the same base station, a contiguous group of cells can be created to form a wide-area network. In this specification, the terms “network” and “system” will be used equivalently.
In systems of the above type, data is communicated by exchange of packets between the mobile stations and base station whilst a connection (management connection or transport connection) is maintained between them. The direction of transmission of packets from the subscriber station to the base station is the uplink (UL), and the direction from the base station to the subscriber station is the downlink (DL). The packets have a defined format which follows a layered protocol applied to the system and its component radio devices. Protocol layers relevant to packets as such are the so-called physical layer (PHY) and media access layer (MAC).
The media access layer is responsible for handling various functions including network access, bandwidth allocation, and maintaining connections. This includes controlling access of the BS and SS's to the radio medium on the basis of “frames” which are the predetermined unit of time in the system, and which are divided in the time and frequency domain into a number of “slots” (see below), and when utilising multiple transmit antennas may also be divided spatially into a number of streams.
A connection between a base station and subscriber station (more precisely, between MAC layers in those devices—so-called peer entities) is assigned a connection ID (CID) and the base station keeps track of CIDs for managing its active connections or service flows. A service flow could represent, for example, a voice call conducted by the user of the MS. In addition, base stations and mobile stations have their own identifying codes (BSID for the BS, MS MAC address or basic CID for the MS).
Various physical layer implementations are possible in an IEEE 802.16 network, depending on the available frequency range and application; for example, a time division duplex (TDD) mode and a frequency division duplex (FDD) mode as described below. The PHY layer also defines the transmission technique such as OFDM (orthogonal frequency division multiplexing) or OFDMA (orthogonal frequency division multiple access), which techniques will now be outlined briefly.
In OFDM, a single data stream is modulated onto N parallel sub-carriers, each sub-carrier signal having its own frequency range. This allows the total bandwidth (i.e. the amount of data to be sent in a given time interval) to be divided over a plurality of sub-carriers thereby increasing the duration of each data symbol. Since each sub-carrier has a lower information rate, multi-carrier systems benefit from enhanced immunity to channel induced distortion compared with single carrier systems. This is made possible by ensuring that the transmission rate and hence bandwidth of each subcarrier is less than the coherence bandwidth of the channel. As a result, the channel distortion experienced on a signal subcarrier is frequency independent and can hence be corrected by a simple phase and amplitude correction factor. Thus the channel distortion correction entity within a multicarrier receiver can be of significantly lower complexity of its counterpart within a single carrier receiver when the system bandwidth is in excess of the coherence bandwidth of the channel.
An OFDM system uses a plurality of sub-carrier frequencies (subcarriers) which are orthogonal in a mathematical sense so that the sub-carriers' spectra may overlap without interference due to the fact they are mutually independent. The orthogonality of OFDM systems removes the need for guard band frequencies and thereby increases the spectral efficiency of the system. OFDM has been proposed and adopted for many wireless systems. In an OFDM system, a block of N modulated parallel data source signals is mapped to N orthogonal parallel sub-carriers by using an Inverse Discrete or Fast Fourier Transform algorithm (IDFT/IFFT) to form a signal known as an “OFDM symbol” in the time domain at the transmitter. Thus, an “OFDM symbol” is the composite signal of all N sub-carrier signals. At the receiver, the received time-domain signal is transformed back to frequency domain by applying Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) algorithm. Not all of the subcarriers are available to carry data; for example, in an IEEE802.16-2004 fixed WiMAX system employing 256 subcarriers, 192 may be available for data, 8 used as pilot subcarriers for channel estimation and synchronisation, and the remainder used as guard band subcarriers.
OFDMA (Orthogonal Frequency Division Multiple Access) is a multiple access variant of OFDM. It works by assigning a subset of the sub-carriers to an individual subscriber. This allows simultaneous transmission from several users leading to better spectral efficiency. However, there is still the issue of allowing bi-directional communication, that is, in the uplink and download directions, without interference. In order to enable bi-directional communication between two nodes, two well known different approaches exist for duplexing the two (forward or downlink and reverse or uplink) communication links to overcome the physical limitation that a device cannot simultaneously transmit and receive on the same resource medium. The first, frequency division duplexing (FDD), involves operating the two links simultaneously but on different frequency bands by subdividing the transmission medium into two distinct bands, one for DL and the other for UL communications. The second, time division duplexing (TDD), involves operating the two links on the same frequency band, but subdividing the access to the medium in time so that only the DL or the UL will be utilizing the medium at any one point in time. Although both approaches have their merits and the IEEE802.16 standard incorporates both an FDD and TDD mode, the remainder of this description will mainly refer to the TDD mode.
A variant of OFDMA, with which the present invention is particularly concerned, is scaleable OFDMA or SOFDMA. In SOFDMA the FFT size, or number of used subcarriers, is scaled based on the bandwidth of the channel that the system is to occupy. The IEEE802.16e specification defines FFT sizes of 128, 512, 1024 and 2048, so that the SOFDMA PHY can be used in channels ranging from 1.25 MHz to 20 MHz without having to significantly alter the subcarrier spacing, which can be optimized based on the propagation channel conditions (i.e mobility and fading) In addition, OFDMA provides a number of “subcarrier allocation” schemes that define how the physical subcarriers are grouped into logical subchannels. One method of subcarrier allocation is called frequency diverse transmission, where a logical subchannel includes subcarriers distributed over the whole frequency range (this is called Full Usage of Subcarriers or FUSC) or distributed within some subset of the subcarriers (Partial Usage of Subcarriers or PUSC). Another subcarrier allocation method—Band Adaptive Modulation and Coding, or AMC, forms subchannels by grouping physically-adjacent subcarriers. The same frame can employ both techniques within separate “zones” in the time dimension. Resources in an OFDMA system are allocated in units of slots as mentioned above. Each slot consists of one subchannel over one, two or three OFDM symbols, depending on the subcarrier allocation scheme used.
The OFDMA PHY also encompasses the modulation and forward error correction (FEC) coding techniques used in the various bursts. Typically the type of modulation and coding rate will depend on the range of the user from the cell site and the signal propagation environment, that is, on how strongly signals from the BS are received at the MS, as this determines the data rate achievable between the BS and MS. One measure of this is the signal to noise-plus-interference ratio experienced by each user. For users with high SINR, quadrature amplitude modulation with 64 levels (64QAM) with high rate convolutional turbo coding (e.g. ⅚) can be used. The BS may employ a more robust QAM (16QAM) and/or code rate for users with lower SINR, and proceeding further to QPSK and/or lower code rates for users with even lower SINR. The BS can use a different modulation technique for each user's downlink and uplink bursts. By selecting an appropriate modulation technique, errors in transmission can be minimized and link efficiency optimized. FIG. 1 illustrates the TDD frame structure used in the OFDMA physical layer mode of the IEEE802.16e-2005 standard (WiMAX). The OFDMA physical layer divides the available OFDM symbols and component subcarriers (see FIG. 1) into distinct logical and physical subchannels using the above subcarrier allocation techniques, allowing multiple bursts to co-exist in each time interval. Each frame is divided into DL and UL subframes, each being a discrete transmission interval. On the downlink DL, a single burst may be shared by several users (subscriber stations) but on the uplink UL, each burst generally corresponds to a single user. In a WiMAX system the DL subframe can contain zones for FUSC, PUSC and AMC and the UL subframe can contain zones with PUSC or AMC. In FIG. 1, the frame can be considered to occupy a given length of time and a given frequency band, the time dimension being denoted in FIG. 1 by “OFDMA symbol number”, and the frequency dimension by “subchannel logical number” (each subchannel is a set of the sub-carriers referred to above). The subframes are separated by a Transmit/Receive and Receive/Transmit Transition Guard interval or Gap (TTG and RTG respectively). The TTG and RTG allow time for the BS and MSs to switch between receive and transmit modes. Each DL subframe starts with a preamble followed by the Frame Control Header (FCH), the DL-MAP, and, if present, the UL-MAP. The FCH contains a DL Frame Prefix (DLFP) to specify the burst profile and the length of the DL-MAP. The DLFP is a data structure transmitted at the beginning of each frame and contains information regarding the current frame; it is mapped to the FCH. DL allocations can be broadcast, multicast and unicast and they can also include an allocation for another BS rather than a serving BS.
The DL-subframe includes a broadcast control field with a DL-MAP and UL-MAP, by which the BS informs the receiving device of the frame structure. The MAP is a map of bandwidth allocation in the frame and also contains other PHY signalling related messages. It consists of Information Elements (MAP_IEs) as shown in FIG. 1, each containing a connection ID. The MAP_IEs inform mobile stations to which burst(s) their connections have been assigned to transmit and receive information. Thus, in a TDD and FDD mode network, bandwidth allocation means the allocation of resources (slots) within frames.
Each DL burst has a “2-D” structure, having a defined extent in both frequency and time dimensions. Thus, the MAP_IE has to inform the MS concerned of not only the part of the frequency spectrum (frequency band), but also the portion of the subframe duration, in which the burst is placed. This information is provided in the form of a subchannel offset (frequency) relative to the 0-th logical subchannel, and a symbol offset (time) relative to the start of the subframe. In the example of FIG. 1, each MS is allocated a 2-D burst within the DL subframe, for transmitting data on the downlink from the BS to the MS. As for the uplink, current WiMAX standards define UL allocations as extending across the whole duration of the relevant zone, which may be the whole subframe if only 1 zone is defined, roughly as illustrated in FIG. 1, but with a snake-like pattern (not shown) such that the allocation to a single connection may carry over to the next frequency band. Thus, in general definition of the UL allocation is simpler than defining the DL allocation as only a duration parameter is required, and in the case of AMC, also an offset in terms of slots from the previous allocation. Respective MAP_IEs link the bursts to respective connections of mobile stations, as indicated by arrows in the Figure. In the case of services such as video streaming, the amount of resource allocation on the downlink may need to be many times as large as that on the uplink.
FIG. 1 shows an example in which seven users are allocated one burst each within both DL and UL subframes, in which the signalling via MAP_IEs is already somewhat complex, but a practical system may need to serve more users concurrently in the same frame, or a single user may occupy more than burst. Thus, the number of MAP_IEs may grow large, increasing the proportion of the DL subframe taken up by controlling signalling and reducing the proportion available for data.
Accordingly, there is a need to improve the signalling mechanism in systems of the above type, especially if they are extended to support larger system bandwidths.